Radial compressor impeller including a shroud and aerodynamic bearing between shroud and housing

ABSTRACT

A compressor for a heat pump circuit and/or a refrigerating system circuit, including a housing and a rotor rotatably supported around a rotation axis, the housing being situated at least partially on the circumference of the rotor, the rotor including at least one hub and at least one blade situated radially on the outside of the hub, the blade being designed to convey a main fluid flow, the rotor including a shroud situated radially on the outside of the blade, the shroud being situated radially spaced apart from the housing, a bearing structure being provided radially on the outside of the shroud, the bearing structure is designed to form a bearing fluid flow between the shroud and the housing in order to form a fluid-dynamic bearing for supporting the rotor in the housing.

FIELD

The present invention relates to a compressor for a heat pump circuitand/or a refrigerating system circuit, including a housing and a rotorrotatably supported around a rotation axis, the housing being situatedat least partially on the circumference of the rotor, the rotorincluding at least one hub and at least one blade situated radially onthe outside of the hub, the blade being designed to convey a main fluidflow, the rotor including a shroud situated radially on the outside ofthe blade, the shroud being situated radially spaced apart from thehousing, a bearing structure being provided radially on the outside ofthe shroud, the bearing structure being designed to form a bearing fluidflow between the shroud and the housing in order to form a fluid-dynamicbearing for supporting the rotor in the housing.

BACKGROUND INFORMATION

Compressors for heat pump circuits and/or refrigerating system circuitshave a rotor which is rotatably supported with the aid of rollerbearings or plain bearings and is driven by a drive unit. Thecompressors are thereby designed to apply pressure to a fluid from aninput side toward an output side and thus to compress the fluid.

It is an object of the present invention to provide an improvedcompressor which has a particularly good support and at the same timemay be manufactured particularly cost-efficiently.

SUMMARY

According to the present invention, it has been found that an improvedcompressor may be provided, that the compressor includes a housing and arotor rotatably supported around a rotation axis. In an exampleembodiment, the housing is situated at least partially on thecircumference of the rotor. The rotor has at least one hub and at leastone blade situated radially on the outside of the hub. The blade isdesigned to convey a main fluid flow. The rotor has a shroud situatedradially on the outside of the blade. The shroud is situated radiallyspaced apart from the housing. A bearing structure is provided radiallyon the outside of the shroud and is designed to form a bearing fluidflow between the shroud and the housing to form a fluid-dynamic bearingfor supporting the rotor in the housing.

This design may have the advantage that a fluid-dynamic support of therotor in the housing may be provided and thus conventional plainbearings and roller bearings may be omitted. Thus, a particularly quietsupport of the rotor is provided which is particularly cost-efficientand at the same time also has a particularly long service life.

In another specific embodiment, the rotor has an input side and anoutput side. The blades are designed to convey the main fluid flow fromthe input side to the output side. The bearing structure is designed toconvey the fluid flow from the output side to the input side. In thisway, it may be ensured that, at a pressure increase between the inputside and the output side in the main fluid flow, this main fluid flowlifts the bearing fluid flow and thus a reliable fluid-dynamic supportof the rotor is ensured. In addition, a particularly reliable supportmay thereby be ensured even at low rotational speeds of the rotor.

In another specific embodiment, the input side is situated radially onthe inside and the output side is situated radially on the outside ofthe rotor, the bearing structure being designed to be at least partiallyhelical. This design has the advantage that a particularly smoothbearing fluid flow may be provided, which rotates in the circumferentialdirection and is also conveyed in the axial direction in the directionof the input.

In another specific embodiment, the bearing structure includes a sealingelement, the sealing element being situated between the shroud and thehousing, the sealing element being designed to delimit the bearing fluidflow in the axial direction. In this way, it may be ensured that thecompressor has a particularly high efficiency and the bearing fluid flowdoes not unnecessarily reduce the delivery volume of the compressor.

It is hereby particularly advantageous if the sealing element isdesigned as a labyrinth seal.

In another specific embodiment, the bearing structure is designed in afishbone pattern and/or the bearing structure has a surface texture (forexample, according to EN ISO 25178, previously called roughness) in therange from 1 Rz through 60 Rz. Thus, the bearing structure may becost-efficiently designed to be flat.

In another specific embodiment, the bearing structure has at least onerecess and/or one bulge which is/are situated obliquely or transverselyto the circumferential direction of the hub. In this way, a particularlyhigh circumferential speed of the bearing fluid flow may be reached.Thus, a particularly stable support of the rotor in the housing may beensured.

In another specific embodiment, the rotor has a second hub, at least onesecond blade being provided situated radially on the outside of thesecond hub. The second blade is designed to convey a second main fluidflow. The second hub is coupled to the hub via a shaft. The rotorincludes a second shroud situated radially on the outside of the secondblade. The second shroud is situated radially spaced apart from thehousing. The housing encompasses the second shroud at least partially onthe circumferential side. A second bearing structure is providedradially on the outside of the second shroud and is designed to providea second bearing fluid flow between the second shroud and the housing.This design has the advantage that the rotor is axially fixed in itsdefined position by the two structures situated diametrically oppositewithout second bearing structures having to be provided for thispurpose.

It is thereby particularly advantageous if the second bearing structureand the bearing structure are designed axis-symmetrically to an axis ofsymmetry situated between the two hubs. It may thus be prevented thatdifferent axial bearing forces are generated by the bearing structureand the second bearing structure, which would result in an unbalancedorientation of the rotor in the compressor.

In another specific embodiment, at least one magnet is situated betweenthe two hubs, the magnet being connected in a torque-locked manner withthe shaft. At least one coil ring is provided radially on the outside ofthe shaft to provide an alternating magnetic field, the alternatingmagnetic field being designed to engage in operative connection with themagnet in order to generate a rotational movement of the rotor. Thus,the rotor may be driven particularly easily.

The present invention will be subsequently explained in greater detailbased on the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional view through a compressor accordingto a first specific embodiment.

FIG. 2 shows a section of the sectional view shown in FIG. 1.

FIG. 3 shows a sectional view through the compressor shown in FIGS. 1through 2 along a section plane A-A shown in FIG. 1.

FIG. 4 shows a schematic sectional view through a compressor accordingto a second specific embodiment.

FIG. 5 shows a schematic sectional view through a compressor accordingto a third specific embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows a sectional view through a compressor 10 according to afirst specific embodiment and FIG. 2 shows a section of the sectionalview shown in FIG. 1. FIG. 3 shows a sectional view through compressor10 shown in FIGS. 1 through 2 along a section plane A-A shown in FIG. 1.Compressor 10 includes a rotor 15 and a housing 20. Housing 20 includesa first housing part 25 which is situated on the left in FIGS. 1 and 2.Housing 20 further includes a second housing part 30 situated on theright in FIG. 1. Rotor 15 is coupled to a drive unit 35. Rotor 15includes a shaft 40 which is connected to drive unit 35. Shaft 40 isthereby rotatable around a rotation axis 45.

Compressor 10 has an input side 50 and an output side 55. Input side 50is designed as a single channel in the specific embodiment, input side50 then branching into two feed channels 51. The two feed channels 51are thus switched in parallel with respect to their flow. It is, ofcourse, also possible that compressor 10 has multiple different inputsides 50 with corresponding feed channels 51 separated from each other.Thus, feed channels 51 may also be switched in series with respect totheir flow, output side 55 of a first rotor section 110 emptying intoinput side 50 of a second rotor section 115. Rotor 15 is designed toconvey a fluid 60 from input side 50 to output side 55 and to therebyincrease a pressure p₁ prevailing at the input side to a pressure p₂prevailing at the output side. Fluid 60 may thereby be a coolant, forexample, CO₂, R-134a, or R-410a. Other fluids are, of course, alsopossible. Here, fluid 60 is present in its gaseous phase or its liquidphase. Compressor 10 may be a turbo compressor for a refrigeratingsystem circuit and/or heat pump circuit. Of course, another applicationis also possible. Thus, it is possible to use compressor 10 in a heatingcircuit including a solar collector and to convey the fluid present inthe heating circuit with the aid of compressor 10.

Rotor 15 has first rotor section 110 situated on the left of drive unit35 and second rotor section 115 situated on the right of drive unit 35.First rotor section has a first hub 65, first blades 70, and a firstshroud 75. First blades 70 are thereby situated radially on the outsideof first hub 65 and extend from radially inside to radially outside.First blades 70 are thereby situated on first hub 65 at a uniformdistance from each other in the circumferential direction. First shroud75 is joined radially on the outside with first blades 70. First shroud75 is situated radially spaced apart from first housing part 25. Firsthousing part 25 encompasses first shroud 75 on the circumferential sideand is formed on an inner first housing surface 76 facing shroud 75,which corresponds to an outer circumferential surface 77 of first shroud75. Due to the spaced arrangement, a first gap 80 with a gap width s₁ isprovided between first housing part 25 and first shroud 75. First shroud75 and first hub 65 delimit a first conveying channel 85. Due to thecone-shaped design of first hub 65 and the likewise cone-shaped designof first shroud 75, first conveying channel 85 runs in the axialdirection from input side 50 to drive unit 35 radially from the insideto the outside, and has a cross section tapering radially outwardly.

First blades 70 are thereby designed to suction fluid 60 radiallyinwardly and to convey it during operation in the axial direction in thedirection of output side 55 or drive unit 35. In order to furtherincrease the pressure, rotor 15 is designed as a radial compressor andconveys fluid 60 radially from the inside to the outside, pressure pincreasing from input side 50 toward output side 55. To prevent animbalance of rotor 15, first hub 65 and first shroud 75 are designed tobe rotationally symmetrical to rotation axis 45. First blades 70 arefurther situated on first hub 65 at uniform spacing in thecircumferential direction.

Rotor 15 has a second hub 90, second blades 95, and a second shroud 100.Second hub 90 is thereby situated on the right side, diametricallyopposite to hub 65 situated on the left side. Second blades 95 areprovided radially on the outside of second hub 90. Second shroud 100 isconnected to second blades 95 radially on the outside of the ends ofblades 95 diametrically opposite to second hub 90. Second shroud 100 isspaced apart from second housing part 30 via a second gap 105 with a gapwidth s₂. An inner second housing surface 108, which faces an outercircumferential surface 107 of second shroud 100, is formedcorresponding to outer circumferential surface 107 of second shroud 100.Second hub 90 has a conical shape like second shroud 100. Second shroud100 and second hub 90 delimit a second conveying channel 106. Secondconveying channel 106 is guided radially from the inside radially to theoutside in an axial direction from input side 50 toward drive unit 35.Second conveying channel 106 is also designed to taper from input side50 toward output side 55. It is, of course, also possible that conveyingchannels 85, 106 also have a constant or expanding cross section. Toprevent an imbalance of rotor 15, second hub 90 and second shroud 100are also provided rotationally symmetrically to rotation axis 45. Secondblades 95 are further situated on second hub 90 at a uniform spacing inthe circumferential direction. Second blades 95 are also used like firstblades 70 for the purpose of conveying fluid 60 from input side 50toward output side 55 through second conveying channel 106 and therebyapply pressure p to fluid 60.

In the specific embodiment, rotor section 110 situated on the left sideof drive unit 35 is formed axially symmetrically to second rotor section115 situated on the right side of the drive unit and to an axis ofsymmetry 120 situated between both rotor sections 110, 115. Each rotorsection 110, 115 is connected to an assigned feed channel 51 of inputside 50.

Of course, an asymmetrical design of rotor 15 is also possible. Ifcompressor 10 has multiple input sides 50, then, for example, one inputside 50 may each be assigned to each rotor section 110, 115. Due to anasymmetrical design of rotor 15, rotor 15 may be adjusted to differentinput sides 50.

Drive unit 35 has at least one magnet 125 which is situated between bothrotor sections 110, 115 and is connected in a torque-locked manner toshaft 40. Further, drive unit 35 includes a coil ring 130 includingmultiple coils 155 and which encompasses the circumferential side ofshaft 40 in the area of magnets 125. Coil ring 130 is connected to acontrol unit 140 via a connection 135. Control unit 140 is connected toan energy source 150 via a second connection 145. Control unit 140 isdesigned to energize the coils 155 situated in coil ring 130 in such away that an alternating magnetic field is provided by coil ring 130which engages in operative connection with magnets 125 and causes arotation of shaft 40 in order to shift rotor 15 into a rotation.

When rotor 15 rotates around rotation axis 45, then a first main fluidflow 160 is conveyed by first blades 70 from input side 50 to outputside 55 via first conveying channel 85. First main fluid flow 160 isguided due to the design of first blades 70 radially from inside tooutside and pressure P₂ is applied in the process. Pressure P₂ at outputside 55 is thus higher than at input side 50.

In second rotor section 115, the delivery is carried out analogous tofirst rotor section 110. In second rotor section 115, a second mainfluid flow 161 is conveyed with the aid of second blades 95 in the axialdirection of drive unit 35 and radially from inside to outside and thepressure is applied.

Due to the pressure difference between output side 55 and input side 50,fluid 60, compressed in output side 55, flows into first and second gap80, 105 as first and second bearing fluid flow 165, 166 between shrouds75, 100 and housing parts 25, 30. Gap width s₁, s₂ is thereby selectedin such a way that bearing fluid flow 165, 166 between housing part 25,30 and shroud 75, 100 is smaller than main fluid flow 160, 161. The flowdirection of bearing fluid flow 165, 166 is from output side 55 in thedirection of input side 50.

A bearing structure 170, 175 is provided circumferentially on shroud 75,100 on an outer circumferential surface facing housing part 25, 30.Bearing structure 170, 175 accelerates bearing fluid flow 165, 166flowing into gap 80, 105 in the direction of rotation of rotor 15. Afluid film 176 thereby forms between housing part 25, 30 and shroud 75,100. Bearing structure 170, 175 may thereby be designed differently inorder to accelerate bearing fluid flow 165, 166 in the circumferentialdirection. Thus, bearing structure 170, 175 may be provided with asurface texture. In FIGS. 1 through 3, the acceleration is generatedwith the aid of the surface texture of shroud 75, 100. Depending on thespeed, a surface texture in the range of 1 Rz through 60 Rz is therebysufficient. It is also alternatively possible that surface structure170, 175 has bulges (compare FIG. 5) and/or recesses (compare FIG. 4),which are designed to convey bearing fluid flow 165, 166 in thecircumferential direction. Thus, bearing fluid flow 165, 166 has a speedcomponent in the axial direction and also in the circumferentialdirection, the speed component prevailing in the circumferentialdirection.

If bearing fluid flow 165 is brought to a predefined speed in thecircumferential direction by rotation of rotor 15, a pressure cushion185 of fluid film 176 or of bearing fluid flow 165, 166 builds up atfirst/second shroud 75, 100 with a bearing force P₁, P₂. The curveddesign of shroud 75, 100 and housing 20 has as a consequence thatbearing force P₁, P₂ runs obliquely to individual axes of a coordinatesystem 190. Coordinate system 190 is designed, for example, as a rightangle coordinate system and is to be used for facilitated directionalreference of the forces. Thus, bearing force P₁, P₂ has a bearing forceP_(x1), P_(x2) running in the axial direction x and a bearing forceP_(y1), P_(y2) running perpendicularly to rotation axis 45 and to thex-axis. Bearing forces P_(y1), P_(y2) in the y-direction are therebyoriented counter to a weight force F of rotor 15. If bearing forcesP_(y1), P_(y2) in the y-direction or pressure cushion 185 are strongenough, then rotor 15 lifts and is exclusively supported via pressurecushion 185. Thus, bearing fluid flow 165, 166 forms a fluid-dynamicfluid bearing 180 between first shroud 75 and first housing part 25 andbetween second shroud 100 and second housing part 30, by which rotor 15may be supported contact-free in housing 20. This takes place, inparticular, if gap width s₁, s₂ of gap 80, 105 is at every point of gap80, 105 in the range from 1 μm through 30 μm, preferably between 1 μmthrough 20 μm during operation of compressor 10.

Due to the weight of rotor 15 or also due to other influences, rotationaxis 45 is situated offset to a housing axis 195, for example, in thedirection of weight force F. Housing axis 195 thereby runs on the x-axisof coordinate system 190. Due to the offset of rotor 15, gap width s₁,s₂ also differs in the circumferential direction during operation ofcompressor 10, gap width s₁, s₂ being smaller on the under side than onthe upper side of rotor 15.

During the start up or the acceleration of rotor 15 to operatingrotational speed, i.e., when combined bearing forces P_(y1), P_(y2) inthe y-direction are smaller than weight force F, the underside ofbearing structure 170, 175 contacts housing part 25, 30. During thestart up, bearing structure 170, 175 forms, together with housing parts25, 30 respectively, a plain bearing in order to support rotor 15 inhousing 20.

Due to the symmetrical design of shrouds 75, 100 and the assignedhousing parts 25, 30, and due to the self-adjusting gap widths s₁ ands₂, the axial bearing forces P_(x1), P_(x2) of both rotor sections 110,115 cancel each other out since the bearing forces are directed in thediametrically opposite direction due to bearing fluid flow 165, 166flowing away from axis of symmetry 120 on both sides. Thus, no secondaxial fixing of rotor 15 in housing 20 is necessary.

After flowing through gap 80, 105, bearing fluid flow 165, 166 is suckedagain into the input side of rotor 15 and compressed again together withmain fluid flow 160, 161.

Due to the brushless configuration of drive unit 35 and the situation ofcoil ring 130 spaced apart from shaft 40, rotor 15 may be reliablysupported in housing 20 with the aid of bearing fluid flow 165 and anoffset of rotation axis 45 of rotor 15 to a housing axis 195, which runsparallel to rotation axis 45, may be compensated for at the same time.

Due to fluid bearing 180, additional plain or roller bearings may beomitted for supporting rotor 15 so that compressor 10 is designedparticularly cost efficiently. In addition, a particularly simplesupport may be provided, in particular for a particularly fast rotatingrotor 15. Due to the omission of plain or roller bearings, compressor 10is designed to be smaller over all, so that compressor 10 has a morecompact installation space requirement.

Due to the provision of two rotor sections 110, 115 symmetrical to axisof symmetry 120, additional bearing systems may be completely omitted.In addition, this design has a particularly high delivery rate.

FIG. 4 shows a schematic sectional view through a compressor 200according to a second specific embodiment. Rotor 15 is represented insection above rotation axis 45 and in a top view below rotation axis 45.Compressor 200 is designed generally identical to compressor 10 shown inFIG. 1. Deviating therefrom, bearing structure 175 has additionalrecesses 205 situated in a fishbone pattern and situated on shroud 75,100 at uniform spacing on the circumferential side.

It is pointed out here that rotor 15 in FIG. 4 is designed to be axiallysymmetrical to axis of symmetry 120, and recesses 205 are also providedon second rotor section 115 situated on the right side (not shown). Inaddition, recesses 205 may, of course, also be provided as bulges whichextend radially outwardly in the direction of housing part 25.

Bearing structure 175 in the specific embodiment is situated in a singlerow on shroud 75, 100. It is, of course, also possible that multiplerows of bearing structure 175 are provided as recesses 205 or bulgessituated in a fishbone pattern on the circumferential side of outercircumferential surface of shroud 75, 100 facing housing part 25, 30.Recesses 205 have one first recess section 206 and one second recesssection 207. Recess sections 206, 207 enclose an opening angle α. Theopening angle is smaller than 180°. Recess sections 206, 207 aresituated in such a way that recesses 205 are open toward the rotationaldirection of rotor 15. Thus, a particularly high circumferential speedmay be induced in bearing fluid flow 165, so that a particularly stablepressure cushion may be formed by bearing fluid flow 165 in the area ofrecesses 205, which pressure cushion supports rotor 15 particularlywell.

FIG. 5 shows a sectional view through a compressor 300 according to athird specific embodiment, rotor 15 being shown in a top view.Compressor 300 is designed generally identical to compressors 10, 200shown in FIGS. 1 through 4. Deviating therefrom, bearing structure 175has bulges 305 which are situated helically on shroud 75 on thecircumferential side. Bulges 305 are thereby designed to be blade like.Alternatively, bulges 305 may be connected in the circumferentialdirection to form a screw. Thus, during a counter-clockwise rotation,bearing fluid flow 165 may be conveyed particularly well in thedirection of a sealing element 310 of bearing structure 170, 175situated radially inside on the input side. Sealing element 310 isdesigned in the specific embodiment as a labyrinth seal, whereby afriction contact between rotor 15 and housing 20 may be prevented. Aparticularly high efficiency of compressor 300 is thereby ensured. Inaddition, sealing element 310 has the advantage that bearing fluid flow165 may be conveyed from output side 55 to input side 50 and may besimultaneously accelerated in the circumferential direction due to thehelical design of bulges 305. Simultaneously, bearing fluid flow 165backs up in front of sealing element 310 so that a pressure p_(s) may bemaintained particularly high within first gap 80. A stable support maythereby already be ensured even at low rotational speeds of rotor 15.

First/second gap 80, 105 is designed to taper from output side 55 towardinput side 50 in the specific embodiment. It is, of course, alsopossible that gap 80, 105 has a constant gap width s₁, s₂ across gap 80,105.

It is pointed out, that sealing element 310 may be situated also atanother position on the circumferential side on housing part 25, 30 orrotor 15 instead of on the input side. It is, of course, also possiblethat the sealing element is provided on an embodiment of rotor 15 shownin FIG. 1 or 2. It is also possible that sealing element 310 is omitted.

Bearing structure 175 is provided as an example in FIGS. 1 through 4. Itis, of course, also possible that bearing structures having anotherdesign are provided; however, it is thereby essential that a bearingfluid flow 165 is lifted up by bearing structure 175 which provides afluid-dynamic bearing point between rotor 15 and housing 20 in order tobe able to omit plain or roller bearings for supporting rotor 15.

Drive unit 35 is an example in the specific embodiment. It is, ofcourse, also possible that drive unit 35 may be designed differently.Drive unit 35 shown in FIGS. 1 through 5 has, however, the advantagethat, due to the noncontact design of drive unit 35 between shaft 40 andcoil ring 130, shaft 40 is displaceable in the radial direction by gapwidth s₁, s₂ so that, depending on the load of rotor 15, bearing fluidflow 165 may support rotor 15 in the axial and also in the radialdirection, regardless of the orientation of compressor 10, 200, 300.

1-10. (canceled)
 11. A compressor for a heat pump circuit orrefrigerating system circuit, comprising: a housing; a rotor rotatablysupported around a rotation axis, the housing being situated at leastpartially on the circumference of the rotor, the rotor including atleast one hub and at least one blade situated radially on an outside ofthe hub, the blade being designed to convey a main fluid flow, the rotorincluding a shroud situated radially on the outside of the blade, theshroud being situated radially spaced apart from the housing; and abearing structure provided radially outside of the shroud, the bearingstructure being designed to form a bearing fluid flow between the shroudand the housing to form a fluid-dynamic bearing for supporting the rotorin the housing.
 12. The compressor as recited in claim 11, wherein therotor includes an input side and an output side, the blade beingdesigned to convey the main fluid flow from the input side toward theoutput side, the bearing structure being designed to convey the bearingfluid flow from the output side toward the input side.
 13. Thecompressor as recited in claim 12, wherein the input side is situatedradially on an inside of the rotor and the output side is situatedradially on an outside of the rotor, the bearing structure being atleast partially helically.
 14. The compressor as recited in claim 12,wherein the bearing structure includes a sealing element, the sealingelement being situated between the shroud and the housing, the sealingelement being designed to delimit the bearing fluid flow in the axialdirection.
 15. The compressor as recited in claim 12, wherein thesealing element is designed as a labyrinth seal.
 16. The compressor asrecited in claim 12, wherein at least one of: i) the bearing structureis designed in a fishbone pattern, and ii) the bearing structure has asurface texture in the range from 1 Rz through 60 Rz.
 17. The compressoras recited in claim 12, wherein the bearing structure has at least oneof: i) at least one recess, and ii) at least one bulge, situatedobliquely or transversely to a circumferential direction of the hub. 18.The compressor as recited in claim 12, wherein the rotor has a secondhub, at least one second blade being situated radially on the outside ofthe second hub, the second blade being designed to convey a second mainfluid flow, the second hub being coupled to the hub via a shaft, therotor including a second shroud situated radially on an outside of thesecond blade, the second shroud being situated radially spaced apartfrom the housing, the housing encompassing the second shroud at leastpartially on a circumferential side, a second bearing structure beingprovided radially outside of the second shroud and being designed toprovide a second bearing fluid flow between the second shroud and thehousing.
 19. The compressor as recited in claim 18, wherein the secondbearing structure and the bearing structure are designed axiallysymmetrically to an axis of symmetry situated between the hub and thesecond hub.
 20. The compressor as recited in claim 19, wherein at leastone magnet is situated on the shaft between the hub and the second hub,the magnet being connected in a torque-locked manner to the shaft, atleast one coil ring being provided radially on the outside of the shaftto provide an alternating magnetic field, the alternating magnetic fieldbeing designed to engage in operative connection with the magnet inorder to induce a rotation of the rotor.